The present invention relates to compositions relating to, and methods for identifying, small conductance (SK) and intermediate conductance (IK), calcium-activated potassium channels. The invention further provides a method to assay for compounds that increase or decrease potassium ion flux through calcium-activated potassium channels.
Calcium-activated potassium currents are found in a wide variety of animal cells such as nervous, muscular, glandular or epithelial tissue and from the immune system. The channels regulating these currents open and allow the escape of potassium as the internal calcium concentration increases. This outward flow of potassium ions makes the interior of the cell more negative, counteracting depolarizing voltages applied to the cell.
Two distinct classes of calcium-activated K+ channels (Kca channels) have been described. Large conductance calcium-activated K+ channels (BK channels) are gated by the concerted actions of internal calcium ions and membrane potential, and have a unit conductance between 100 and 220 pS. Small (SK) and intermediate (IK) conductance calcium-activated K+ channels are gated solely by internal calcium ions, with a unit conductance of 2-20 and 20-85 pS, respectively, and are more sensitive to calcium than are BK channels (for review see Latorre et al., 1989, Ann Rev Phys, 51, 385-399.). In addition, each type of KCs channel shows a distinct pharmacological profile. All three classes are widely expressed, and their activity hyperpolarizes the membrane potential. Members of the BK (Atkinson et al., 1991, Science, 253, 551-555.; Adelman et al., 1992 Neuron, 9, 209-216.; Butler, 1993, Science, 261, 221-224) and SK (Kohler et al., 1996, Science, 273, 1709-1714.) subfamilies have been cloned and expressed in heterologous cell types where they recapitulate the fundamental properties of their native counterparts.
In vertebrate neurons action potentials are followed by an afterhyperpoiarization (AHP) that may persist for several seconds and have profound consequences for the firing pattern of the neuron. Alterations in the AHP have been implicated in seizure activity (Alger et al, J. Physiol. 399:191-205 (1988)) and learning and memory (de Jonge et al., Exp. Br. Res. 80:456-462 (1990)). The AHP is composed of two prominent components, a fast component (fAHP) which mediates spike frequency at the onset of a burst, and a subsequent slow component (sAHP) which is responsible for spike-frequency adaptation (Nicoll, Science 241:545-551 (1988)).
Each component of the AHP is kinetically distinct and is due to activation of different calcium-activated potassium channels. Activation of large-conductance (100-200 picoSiemens (pS)), voltage- and calcium-activated potassium channels (BK channels) underlies the fAHP (Lancaster et al, J. Physiol. 389:187-203 (1987); Viana et al., J. Neurophysiol. 69:2150-2163 (1993)) which develops rapidly (1-2 ms) and decays within tens of milliseconds. The channels underlying the sAHP are small conductance, calcium activated, potassium channels (SK channels) which differ from BK channels, being more calcium-sensitive, are not voltage-gated, and possessing a smaller unit conductance (Lancaster et al., J. Neurosci. 11:23-30 (1991); San. J. Neurophysiol. 74:1772-1776 (1995)).
The fAHP and the sAHP also differ in their pharmacology. The fAHP is blocked by low concentrations of external tetraethylammonium (TEA) and charybdotoxin (CTX), in accord with the pharmacology of the BK channels. Lancaster et al., J. Physiol. 389:187-203 (1987); Viana et al., J. Neurophysiol. 69:2150-2163 (1993), Butler et al., Science 261:221-224 (1993). In contrast, the sAHP is insensitive to CTX, but fall into two classes regarding sensitivity to the bee venom peptide toxin, apamin. For example, in hippocampal pyramidal neurons, the sAHP is insensitive to apamin (Lancaster et al., J. Neurophysiol. 55:1268-1282 (1986)), while in hippocampal interneurons and vagal neurons it is blocked by nanomolar concentrations of the toxin (Sah, J. Neurophysiol. 74:1772-1776 (1995); Zhang et al., J. Physiol. 488:661-672 (1995)).
In addition to its role in neuronal cells, non-voltage gated, apamin-sensitive potassium channels activated by submicromolar concentrations of calcium have also been described from peripheral cell types, including skeletal muscle (Blatz et al., Nature 323:718-720 (1986)), gland cells (Tse et al., Science 255:462-464 (1992); Park, J. Physiol. 481:555-570 (1994)) and T-lymphocytes (Grissmer et al., J. Gen. Physiol. 99:63-84 (1992)).
For example, SK channels have been suggested to represent the apamin receptor found in muscle membrane of patients with myotonic muscular dystrophy. Renaud et al., Nature 319:678-680 (1986)). Also, Grissmer et al. (J. Gen. Physiol. 99:63-84 (1992)) report that CTX insensitive, apamin sensitive calcium-activated potassium channels were identified in a human leukemic T cell line and suggest that calcium-activated potassium channels play a supporting role during T-cell activation by sustaining dynamic patterns of calcium signaling. And in many cells, SK channels are activated as a result of neurotransmitter or hormone action. Haylett et al., in Potassium Channels: Structure, Classification, Function and Therapeutic Potential (Cook, N. S., ed.), pp.71-95, John Wiley and Sons, 1990). Intermediate channels play a role in the physiology of red blood cells.
Intermediate conductance, calcium activated potassium channels have been previously described in the literature by their electrophysiology. The Gardos channel is opened by submicromolar concentrations of internal calcium and has a rectifying unit conductance, ranging from 50 pS at xe2x88x92120 mV to 13 pS at 120 mV (symmetrical 120 mM K+; Christophersen, 1991, J. Membrane Biol., 119, 75-83.). It is blocked by charybdotoxin (CTX) but not the structurally related peptide iberiotoxin (IBX), both of which block BK channels (Brugnara et al., 1995a, J. Membr. Biol., 147, 71-82). Apamin, a potent blocker of certain native (Vincent et al., 1975, J. Biochem., 14, 2521.; Blatz and Magleby, 1986, Nature, 323, 718-720.) and cloned SK channels do not block IK channels (de-Allie et al.,1996, Br. J. Pharm., 117,479-487). The Gardos channel is also blocked by some imidazole compounds, such as clotrimazole, but not ketoconazole (Brugnara et al., 1993, J. Clin. Invest., 92,520-526). The electrophysiological and pharmacological properties of the Gardos channel show that it belongs to the IK subfamily of this invention.
IK channels have been described in a variety of other cell types. Principle cells of the rat cortical collecting duct segregate different classes of K+ channels to the luminal and basolateral membranes. IK channels are present in the basolateral membrane where they promote the recirculation of K+ across this membrane, elevating the activity of the Na++K+-ATPase and thereby Na+ reabsorption into the blood (Hirsch and Schlatter, 1995, Pflxc3xcgers Arch.xe2x80x94Eur. J. Physiol., 449, 338-344.) IK channels have also been implicated in the microvasculature of the kidney where they may be responsible for the vasodilatory effects of bradykinin (Rapacon et al., 1996). In brain capillary endothelial cells, IK channels are activated by endothelin, produced by neurons and glia, shunting excess K+ into the blood (Renterghem et al., 1995, J. Neurochem., 65, 1274-1281). Neutrophil granulocytes, mobile phagocytic cells which defend against microbial invaders, undergo a large depolarization subsequent to agonist stimulation, and IK channels have been implicated in repolarizing the stimulated granulocyte (Varnai et al., 1993, J. Physiol., 472, 373-390.). IK channels have also been identified in both resting and activated human T-lymphocytes. Grissmer et al. 1993, J. Gen. Physiol. 102,601-630 reported that IK channels were blocked by low nanomolar concentrations of charybdotoxin, showed little or no voltage dependence, and were insensitive to apamin. This channel has also been identified in human erythrocytes, where it plays an important role in intracellular volume nomeostasis (Joiner, C. H., 1993. Am. J. Physiol. 264: C251-270 and in smooth muscle (Van Rentergnem, C. et al. 1996, J. Neurochemistry 65,1274-1281.
Thus, it appears that SK and IK channels comprise a subfamily of calcium-activated potassium channels which play key physiological roles in many cell types. Accordingly, given the key role of SK and IK channels in a wide variety of physiological functions, what is needed in the art is the identification of novel SK and IK channel proteins and the nucleic acids encoding them. Additionally, what is needed are methods of identifying compounds which increase or decrease SK and IK channel currents for their use in the treatment or regulation of: learning and memory disorders, seizures, myotonic dystrophies, immune responses, and neurotransmitter or hormone secretions. The present invention provides these and other advantages.
In a first broad context, this invention provides for novel proteins and their corresponding nucleic acids where the proteins are defined as monomers of calcium activated potassium ion channels. The monomers have a molecular weight of between 40 and 80 kDa and have units of conductance of between 2 and 80 pS when the monomer is in the polymeric form as expressed in Xenopus oocytes. In addition, the monomer specifically binds to antibodies generated against SEQ ID NO:30 or 42.
In another aspect, the present invention relates to an isolated nucleic acid encoding at least 15 contiguous amino acids of a calcium-activated potassium channel protein. The SK channel protein has a sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47 and conservatively modified variants of SEQ ID NOS:1, 2, 3, 4, 19, 20, 32, 43 or 47.
In some embodiments, the isolated nucleic acid encodes a calcium-activated potassium channel protein having a conductance of at least 2 pS when expressed in a Xenopus oocyte, a molecular weight of between 40 and 100 kilodaltons (kd), and selectively hybridizes, under stringent hybridization conditions, with SK or IK encoding nucleic acid such as SEQ ID NO:13 in a human genomic library or SEQ ID NO:14 in a rat genomic library. In other embodiments, the isolated nucleic acid encoding the calcium-activated potassium channel protein encodes a protein having a sequence selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47. In preferred embodiments the nucleic acid has a sequence selected from the group consisting of: SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:31, SEQ ID NO:44, and SEQ ID NO:48.
In another aspect, the present invention relates to an isolated calcium-activated potassium channel protein having at least 15 contiguous amino acids of a sequence selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47, and conservatively modified variants of SEQ ID NOS:1, 2, 3, 4, 19, 20, 32, 43, or 47, wherein the variant specifically reacts, under immunologically reactive conditions, with an antibody reactive to a protein selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:30, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47.
In a broad embodiment, the calcium-activated potassium channel protein is defined as having a conductance of at least 2 pS and a molecular weight of between 40 and 100 Kd. In other embodiments, the calcium-activated potassium channel protein has an amino acid sequence selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47.
In another aspect, the present invention is directed to an antibody specifically reactive, under immunologically reactive conditions, to a calcium-activated potassium channel protein, where the protein has a sequence selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4. SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47. In preferred embodiments, the antibody is limited to a monoclonal antibody.
In yet another aspect, the present invention relates to an expression vector comprising a nucleic acid encoding a monomer of a calcium-activated potassium channel where the monomer has a sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47, and conservatively modified variants of SEQ ID NOS:1, 2, 3, 4, 19, 20, 32, 43, or 47 wherein the modified variant is a protein having a conductance of at least 2 pS when expressed in a Xenopus oocyte, a molecular weight of between 40 and 100 kd, and specifically reacts, under immunologically reactive conditions, with an antibody reactive to a full-length protein selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47.
In another aspect, the present invention relates to a host cell transfected with a vector comprising a nucleic acid encoding a monomer of a calcium-activated potassium channel protein where the protein has a sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, SEQ ID NO:47, and conservatively modified variants of SEQ ID NO:1, 2, 3, 4, 19, 20, 32, 43 or 47 wherein the modified variant is a protein having a conductance of at least 2 pS when expressed in a Xenopus oocyte, a molecular weight of between 40 and 100 Kd, and specifically reacts, under immunologically reactive conditions, with an antibody reactive to a full-length protein selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47. Typically, the host cell is cultured under conditions permitting expression of the nucleic acid encoding the calcium-activated potassium channel protein.
In yet a further aspect, the present invention relates to an isolated nucleic acid sequence of at least 15 nucleotides in length which specifically hybridizes, under stringent conditions, to a nucleic acid encoding a calcium-activated potassium channel protein, where the protein is selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47.
In an additional aspect, the present invention is directed to a method for detecting the presence of a calcium-activated potassium channel protein in a biological sample. The method comprises contacting the biological sample with an antibody, wherein the antibody specifically reacts, under immunologically reactive conditions, to an calcium-activated potassium channel protein having a sequence selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47 and allowing the antibody to bind to the protein under immunologically reactive conditions, wherein detection of the bound antibody indicates the presence of the channel protein.
In yet another aspect, the present invention provides a method for detecting the presence, in a biological sample, of a nucleic acid sequence encoding a calcium-activated potassium channel protein of at least 25 amino acids in length. The method comprises contacting the biological sample, under stringent hybridization conditions, with a nucleic acid probe comprising a nucleic acid segment that selectively hybridizes to a nucleic acid encoding the channel protein having a sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47; allowing the nucleic acid encoding the channel protein to selectively hybridize to the probe to form a hybridization complex, wherein detection of the hybridization complex is an indication of the presence of the nucleic acid sequence in the sample. In some embodiments, the hybridization conditions are moderate stringency hybridization conditions. In another embodiment, the calcium activated channel protein is at least 400 amino acid residues in length and when expressed in oocytes has a conductance of at least 2 pS. In a further embodiment, the nucleic acid probes comprises at least 250 contiguous nucleotides encoding a subsequence within the small or intermediate calcium-activated potassium channel protein core region.
In a further aspect, the present invention relates to an isolated calcium-activated potassium channel encoded by a nucleic acid amplified by primers which selectively hybridize, under stringent hybridization conditions, to the same nucleic acid sequence as primers selected from the group consisting of: for hSK1, SEQ ID NO:5 and SEQ ID NO:6; for rSK2 SEQ ID NO:7 and SEQ ID NO:8; for endogenous rSK3, SEQ ID NO:9 and SEQ ID NO:10; for rSK1, SEQ ID NO:11 and SEQ ID NO:12; for hSK2, SEQ ID NO:23 and SEQ ID NO:24; for hSK3, SEQ ID NO:25 and SEQ ID NO:26; and for hlK the following primer pairs will amplify a probe that is selective for identifying hIK1 from a human genomic or cDNA library: 5xe2x80x2 GCCGTGCGTGCAGGATTTAGG 3xe2x80x2 (SEQ ID NO:34) and 5xe2x80x2 CCAGAGGCCAAGCGTGAGGCC 3xe2x80x2 (SEQ ID NO:35) yielding a probe of about 270 bases or 5xe2x80x2 TCCAAGATGCACATGATCCTG 3xe2x80x2(SEQ ID NO:36) and 5xe2x80x2 GGACTGCTGGCTGGGTTCTGG 3xe2x80x2 (SEQ ID NO:37) yielding a probe of about 165 bases. For amplification of a full length hIK1 either of the following two primer pairs will work: 5xe2x80x2 ATGGGCGGGGATCTGGTGCTTG 3xe2x80x2(SEQ ID NO:38) and 5xe2x80x2 CTACTTGGACTGCTGGCTGGGTTC 3xe2x80x2(SEQ ID NO:39) or 5xe2x80x2 ATGGGCGGGGATCTGGTGCTTGG 3xe2x80x2 (includes codon of initiator methionine) (SEQ ID NO:40) and 5xe2x80x2 GGGTCCAGCTACTTGGACTGCTG 3xe2x80x2 (includes stop codon for end of translation) (SEQ ID NO:41).
In yet another aspect, the present invention relates to a method of identifying a compound which increases or decreases the potassium ion flux through a small or intermediate conductance, calcium-activated potassium channel, with the provisio that the compound is not clotrimizole. The method comprises the steps of contacting the compound with a eukaryotic host cell in which has been expressed a nucleic acid encoding a calcium-activated potassium channel having a sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, SEQ ID NO:47 and conservatively modified variants thereof, wherein said conservatively modified variant specifically binds to antibodies specifically reactive with an antigen having an amino acid sequence selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47, have a conductance of at least 2 pS, and a molecular weight between 40 and 100 kilodaltons; and determining the increased or decreased flux of potassium ions through said channel. In preferred embodiments, the increased or decreased flux of potassium ions is determined by measuring the electrical current or flux of ions, or indirectly the change in voltage induced by the change in current or flux of ions, across the cell membrane of said eukaryotic host cell. In a particularly preferred embodiment, the channel protein has a sequence selected from the group consisting of SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47. In another preferred embodiment, the channel protein is recombinant.
In a further aspect, the present invention relates to an isolated eukaryotic nucleic acid encoding a calcium-activated potassium channel protein of at least 400 amino acid residues in length, wherein the calcium-activated channel protein comprises an amino acid sequence having at least 55 to 60% similarity over the length of a core region of a protein selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47 and wherein the channel protein has a conductance of at least 2 pS. In some embodiments, the present invention is directed to the protein encoded by the aforementioned isolated eukaryotic nucleic acid. In other embodiments, the isolated nucleic acid encoding the calcium-activated channel protein has at least 85% sequence similarity over a comparison window of 20 contiguous amino acid residues within the core region.
In a further aspect, the present invention is directed to a vector comprising an isolated eukaryotic nucleic acid encoding a calcium-activated potassium channel protein of at least 400 amino acid residues in length, wherein the channel protein comprises an amino acid sequence having at least 55% similarity over the length of a core region of a protein selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO;3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:32, SEQ ID NO:43, and SEQ ID NO:47, and wherein the channel protein has a conductance of at least 2 pS. Typically, the vector is transfected into a host cell which is cultured under conditions permitting expression of the isolated eukaryotic nucleic acid encoding the channel protein.
In a further aspect, present invention is directed to a method of identifying a compound that increases or decreases the potassium ion flux through a calcium-activated potassium channel. The methods comprises the steps of contacting the compound with a eukaryotic host cell in which has been expressed a calcium-activated potassium channel protein of at least 400 amino acid residues in length, wherein the channel protein has an amino acid sequence having at least 55% similarity over the length of a core region of a protein selected from the group consisting of: SEQ ID NO:1, SEQ ID NO:2, SEQ ID NO:3, SEQ ID NO:4, SEQ ID NO:19, SEQ ID NO:20, SEC ID NO:32, SEQ ID NO:43, and SEQ ID NO:47, and wherein the channel protein has a conductance of at least 2 pS; and determining the increased or decreased flux of potassium ions through the channel protein. In some embodiments the increased or decreased flux of potassium ions is determined by measuring the electrical current across the cell membrane of the eukaryotic host cell.
In another aspect, the present invention provides in a computer system a method of screening for mutations of SK and IK genes, the method comprising the steps of: (i) receiving input of a first nucleic acid sequence encoding a calcium-activated channel protein having a sequence selected from the group consisting of SEQ ID NOS:1, 2, 3, 4, 19, 20, 32, 43, 47 and conservatively modified versions thereof; (ii) comparing the first nucleic acid sequence with a second nucleic acid sequence having substantial identity to the first nucleic acid sequence; and (iii) identifying nucleotide differences between the first and second nucleic acid sequences. In one embodiment, the second nucleic acid sequence is associated with a disease state.
In another aspect, the invention provides in a computer system, a method for identifying a three-dimensional structure of SK and IK proteins, the method comprising the steps of: (i) receiving input of an amino acid sequence of a calcium-activated channel protein or a nucleotide sequence of a gene encoding the protein, the protein having an amino acid sequence selected from the group consisting of SEQ ID NOS:1, 2, 3, 4, 19, 20, 32, 43, 47, and conservatively modified versions thereof; and (ii) generating a three-dimensional structure of the protein encoded by the amino acid sequence. In one embodiment, the amino acid sequence is a primary structure and the generating step includes the steps of forming a secondary structure from the primary structure using energy terms encoded by the primary structure and forming a tertiary structure from the secondary structure using energy terms encoded by said secondary structure. In another embodiment, the generating step includes the step of forming a quaternary structure from the tertiary structure using nianistrophy terms encoded by the tertiary structure. In another embodiment, the method further comprises the step of identifying regions of the three-dimensional structure of the protein that bind to ligands and using the regions to identify ligands that bind to the protein.